Impregnated type cathode assembly, cathode substrate for use in the assembly, electron gun using the assembly, and electron tube using the cathode assembly

ABSTRACT

There is provided an impregnated-type cathode substrate comprising a large particle diameter low porosity region and a small particle diameter high porosity region which is provided in a side of an electron emission surface of the large particle diameter low porosity region and has an average particle diameter smaller than an average particle diameter of the large particle diameter low pore region and a porosity higher than a porosity of the large particle diameter low porosity region, the impregnated-type cathode being impregnated with an electron emission substance.

This application is the national phase of international applicationPCT/JP96/01527, filed Jun. 6, 1996 which designated the U.S.

TECHNICAL FIELD

The present invention relates to an electron tube such as a colorpicture tube, a klystron tube, a traveling wave tube, a gyrotron tube.

BACKGROUND ART

In recent years, a micro-wave electron tube such as a klystron or thelike have had a tendency to exhibit a high output. Particularly, thosetubes which are used in a plasma apparatus for nuclear fusion or aparticle accelerator exhibit an output of a megawatt or more. A muchhigher output is required for those tubes. Meanwhile, there have beendemands for developments in a color picture tube improved in resolutionby increasing scanning lines and a super high frequency responsivepicture tube, and hence, improvements in brightness have been required.Improvements in brightness have also been required for a projectiontube. To respond to these requirements and demands, the emission currentdensity of a current from a cathode must be greatly increased incomparison with a conventional apparatus.

Several conventional electronic tubes such as a color picture tube usedin a color picture receiver require a high voltage supplied to aconvergence electrode, a focus electrode or the like, in addition to ananode voltage. In this case, a problem issues in the aspect of awithstand voltage if a high voltage is supplied from a stem portion ofthe color picture tube. Therefore, a method is adopted in which aresister for a divisional voltage together with an electron gun areincorporated as a electron-gun built-in resister into the color picturetube and in which an anode voltage is divided to supply high voltages toelectrodes, respectively.

Starting from studies made in 1939, developments have been made to usethis tube as an amplifier tube, an oscillation tube, or the like whichcan widely response to an UHF band to a milli wave range. In 1960s,further developments have been started to use a klystron tube for asatellite communication earth station. In 1970s, studies have beenpromoted in view of high efficiency operation of a klystron tube, andproducts with an efficiency of 50% or more have been put into practicaluse including UHF-TV broadcasting. Recently, a klystron tube of a superhigh power has been developed which attains an efficiency of 50 to 70%,a continuous wave output of 1 MW, and a pulse output of 150 MW, and hasbeen used in an accelerator of a super large scale, a plasma heatingapparatus for nuclear fusion studies. A klystron tube can generate ahigh power at a high efficiency, and is therefore used widely in thefield of high power tubes.

A traveling wave tube was invented in 1943 and was completed thereafter.There are various types of traveling wave tubes, such as a spiral type,a cavity coupling type, a cross finger type, a ladder type, and thelikes. A traveling wave tube of a spiral type has been widely used as atransmitting tube to be mounted on an air-plane, an artificial satelliteor the like. A cavity connection type traveling wave tube has beendeveloped for the purpose of compensating for a withstanding powercapacitance of a spiral type, and has been put into practice mainly as atransmitting tube for a satellite communication earth station. Althougha traveling wave tube normally attains an efficiency of about several to20%, a traveling wave tube which attains an efficiency of 50% has beendeveloped for a satellite when electrical potential depression-typecorrector is provided with the traveling wave tube.

Meanwhile, as well-known, a gyrotron tube is an electron tube based onan operation principle of a cyclone maser effect, and is used as a highfrequency high power source which generates a high power milli wave ofseveral tens to several hundreds GHz.

An impregnated-type cathode ensures a higher emission current densitythan an oxide cathode, and has therefore been used as an electron tubefor a cathode ray tube, a traveling wave tube, a klystron tube, agyrotron tube, or the like. Use of an impregnated-type cathode has beenlimited to particular applications such as an HD-TV tube, an ED-TV tube,and the likes, in the field of color picture tubes. However, demands fora large-size CRT and the likes have increased in recent years, and theuse filed of an impregnated-type cathode has been rapidly expanded.

For example, in case of an impregnated-type cathode assembly used inklystron tubes and color picture tubes, the cathode substrate is made ofporous tungsten (W) of a porosity 15 to 20%, and the porous portion ofthis cathode substrate is impregnated with electron emission substancessuch as barium oxide (BaO), calcium oxide (CaO), aluminum oxide (Al₂O₃), and the likes. Further, an iridium (Ir) thin film layer is providedon the electron emission surface of the cathode substrate by a thin filmformation means like a sputtering method, thereby using animpregnated-type cathode assembly coated with iridium.

In this cathode assembly, for example, barium (Ba) and oxygen (O₂)impregnated in the cathode assembly is diffused by an aging step afterthe cathode assembly is mounted in the electron tube, so that dipolelayer is formed on the electron emission surface of the cathode assemblysurface. As a result, a high emission current is enabled.

Although the aging time in an aging step is variously arranged inaccordance with an applied voltage during use of an electron tube as atarget, an dipole layer can be formed in an aging time of about 50 hoursin case of an electron tube used in low voltage operation, for example,with an applied voltage of about 10 kV.

On the contrary, in case of an electron tube used in high voltageoperation, e.g., a super high power klystron tube used with an appliedvoltage of 70 kV, a current of a sufficient current density can bepicked up by aging of a relatively short time period of several tenshours where a current picked up has a pulse width of 5 μs and isrepeated for 500 times for every one second. However, if a current thuspicked up is a direct current, aging requires 500 hours or more to pickup a current of an equal current density.

In case of an electron tube such as a super high power klystron tubeused in high voltage operation, a large amount of gas emitted from acollector is collided with electrons to be ionized at the same time whenan dipole layer is formed by means of aging. Further, these ions collidewith an electron emission surface due to a high voltage, therebybreaking the dipole layer. In this state, the ionized gas has a highenergy. As the amount of gas which collides with the electron emissionsurface increases, the dipole layer of the electron emission surface isbroken seriously. Therefore, an electron tube used in high voltageoperation requires aging of a long time.

In addition, an impregnated-type cathode assembly for a cathode ray tubeis formed to have a compact structure for the purpose of energy saving.Therefore, an impregnated-type cathode assembly for a cathode ray tubehas a limited thickness and a limited diameter which make it difficultto impregnate a sufficient amount of electron emission substance.Generally, the characteristics of the life-time of an impregnated-typecathode are dependent on the amount of evaporation of barium as a maincomponent of electron emission substance. As barium is consumed byevaporation, the monolayer covering late decreases. Electron emissionability decreases in accordance with an increase in the work function.As a result of this, the long life-time characteristic cannot beachieved. This is a large practical problem. From this stand of view, animpregnated-type cathode assembly is desired which can be operated at alow temperature.

In recent years, attentions have been paid to a scandium-based (orSc-based) impregnated-type cathode assembly as such a cathode assemblyfor a cathode ray tube.

The scandium-based impregnated-type cathode assembly described above hasan excellent pulse emission characteristic at a low duty, in comparisonwith an impregnated-type cathode assembly coated with metal, and isexpected to be capable of operating at a low temperature.

However, in this scandium-based impregnated-type cathode assembly whichcan be operated at a low temperature, recovery of lost Sc is slow andthe operation ability at a low temperature is lowered if the cathodeonce receives an ion impact under a condition of a high frequency. Thus,this assembly is not sufficiently practicable.

For example, in case of a type in which a scandium compound is coveredover the surface of the cathode substrate, the surface state changesduring steps of manufacturing a cathode. Operation over a long timeleads to dissipation of scandium and to deterioration in the electronemission characteristic. In addition, the surface of the substrate islocally broken due to ion impacts, and the work function of brokenportions is raised so that the distribution of electron emission becomesnon-uniform.

As a result of Auger surface analysis in a scandium-basedimpregnated-type cathode, it has been determined that scandium on thesurface is lost upon an ion impact and recovery of an excellent densityof electron emission requires a long time, in case of a scandium-basedimpregnated-type cathode.

The followings are examples of a conventional cathode substrate.

Japanese Patent Application KOKAI Publication No. 56-52835 and JapanesePatent Application KOKAI Publication No. 58-133739 disclose a cathodesubstrate in which a cover layer having a porosity of 17 to 30% isprovided on a porous substrate, and this porosity of the cover layer islower than that of the porous substrate. However, in this kind ofcathode substrate, the porosity of the cover layer is arranged to below, and therefore, evaporation of an electron emission substance isrestricted to be low, so that the life-time of the cathode can beelongated. However, under operating condition that ion impacts arestrong as in an electron tube which operates at a high current density,recovery of the structure of the cathode substrate surface is late, sothat excellent results cannot be obtained. Japanese Patent ApplicationKOKAI Publication 58-177484 discloses a cathode substrate containingscandium, which cannot attain sufficient recovery of scandium after anion impact. Therefore, this cathode substrate achieves only aninsufficient low-temperature operation ability. Japanese PatentApplication KOKAI Publication 59-79934 discloses a cathode substrate inwhich a layer containing high melting point metal and scandium is formedon a high melting point metal layer. In this cathode substrate, recoveryof scandium after an ion impact is not sufficient, and therefore, asufficient operation ability at a low temperature cannot be attained.

Japanese Patent Application KOKAI Publication 59-203343 discloses acathode substrate in which a uniform layer containing fine tungsten of0.1 to 2 μm, scandium oxide and electron emission substances is formedon a porous base made of tungsten. This cathode substrate containsscandium, and therefore, can be operated at a low temperature. However,under operating condition that ion impacts are strong, recovery of thestructure of the cathode substrate surface is late, so that excellentresults cannot be obtained. Japanese Patent Application KOKAIPublication 61-91821 discloses a cathode substrate in which a coverlayer made of tungsten and scandium oxide is provided on a poroussubstrate. This cathode substrate contains scandium, and therefore, canbe operated at a low temperature. However, under operating conditionthat ion impacts are strong, recovery of the structure of the cathodesubstrate surface is late, so that excellent results cannot be obtained.Japanese Patent Application KOKAI Publication 64-21843 discloses acathode substrate in which a first formed body having a large averageparticle diameter of, for example, 20 to 15 μm is provided, and a tophead whose average particle diameter is smaller than that of the firstformed body is provided on the first formed body. In this cathodesubstrate, evaporation of an electron emission substance is restrictedto be low, and therefore, the life-time of the cathode can be elongated.However, under operating condition that ion impacts are strong, recoveryof the structure of the cathode substrate surface is late, so thatexcellent results cannot be obtained.

Further, Japanese Patent Application KOKAI Publication 1-161638discloses a cathode substrate in which a layer of scandium compound orscandium alloy is provided on a porous substrate made of high meltingpoint metal. Japanese Patent Application KOKAI Publication No. 3-105827and Japanese Patent Application KOKAI Publication No. 3-25824 disclose acathode substrate in which a layer of a layered structure or of amixture substance is formed on a porous substrate. The layered structureconsists of a mixture layer of tungsten and scandium oxide, and a layerof a scandium supplier, e.g., Sc combined with Re, Ni, Os, Ru, Pt, W,Ta, Mo, or the like. The mixture substance is made of these materials.Japanese Patent Application KOKAI Publication No. 3-173034 discloses acathode substrate in which a layer containing barium and scandium isincluded as an upper layer of a high melting point metal poroussubstrate. Japanese Patent Application KOKAI Publication No. 5-266786discloses a cathode substrate in which, for example, a layered structurecontaining high melting point metal such as a tungsten layer, a scandiumlayer, a rhenium layer and the like is formed on a porous substrate madeof high melting point metal. However, the cathode substrates describedabove cannot ensure sufficient recovery of scandium after an ion impact,the low-temperature operation ability is insufficient. Thus, asufficient ion-impact resistance cannot be attained.

DISCLOSURE OF INVENTION

As has been explained above, a conventional impregnated-type cathodeassembly cannot attain a sufficient ion-impact resistance undercondition of a high voltage and a high frequency. Therefore,deterioration in the electron emission characteristic due to an ionimpact cannot be sufficiently prevented, and hinders improvements inoutputs of an electron tube and in brightness of a picture tube.

In addition, in a scandium-based impregnated-type cathode assembly whichcan be operated at a low temperature, there is a drawback that recoveryof lost Sc is late and the operation ability at a low temperature isdeteriorated if the cathode once receives an ion impact under conditionof a high frequency. Thus, this cathode assembly is not sufficientlypracticable.

The present invention has been made in view of problems as describedabove, and has a first object of providing an improved impregnated-typecathode substrate with a high performance and a long life-time, whichexhibits a sufficient ion-impact resistance and an excellent electronemission under condition of a high voltage and a high frequency.

The present invention has a second object of obtaining an excellentimpregnated-type cathode assembly with use of an improvedimpregnated-type cathode substrate.

The present invention has a third object of obtaining an excellentelectron gun assembly with use of an improved impregnated-type cathodesubstrate.

The present invention has a fourth object of obtaining an excellentelectron tube with use of an improved impregnated-type cathodesubstrate.

The present invention has a fifth object of providing a preferred methodof manufacturing an impregnated substrate according to the presentinvention.

Firstly, the present invention provides an impregnated-type cathodesubstrate comprising a large particle diameter low porosity region and asmall particle diameter high porosity region which is provided in a sideof an electron emission surface of the large particle diameter lowporosity region and has an average particle diameter smaller than anaverage particle diameter of the large particle diameter low porosityregion and a porosity higher than a porosity of the large particlediameter low porosity region, said impregnated-type cathode beingimpregnated with an electron emission substance.

Secondly, the present invention provides a method of manufacturing animpregnated-type cathode substrate according to the first presentinvention, characterized by comprising:

a step of forming a porous sintered body to form a large particlediameter low porosity region;

a step of obtaining a porous cathode pellet by forming a small particlediameter high porosity region in an electron emission surface side ofthe porous sintered body, said small particle diameter high porosityregion having an average particle diameter smaller than that of thelarge particle diameter low porosity region and a porosity higher thanthe porosity of the large particle diameter low porosity region;

a step of cutting or punching the porous pellet, thereby to form aporous cathode substrate; and

a step of impregnating the porous cathode substrate with an electronemission substance.

Thirdly, the present invention provides a method of manufacturing animpregnated-type cathode substrate according to the first aspect of theinvention, characterized by comprising:

a step of forming a porous sintered body to form a large particlediameter low porosity region;

a step of obtaining a porous cathode pellet by forming a small particlediameter high porosity region in an electron emission surface side ofthe porous sintered body, said small particle diameter high porosityregion having an average particle diameter smaller than that of thelarge particle diameter low porosity region and a porosity higher thanthat of the large particle diameter low porosity region;

a step of providing a filler selected from a group of metal andsynthetic resin having a melting point of 1200° C. or less, in theelectron emission surface side of the porous cathode pellet;

a step of heating the porous cathode pellet provided with the filler, ata temperature at which the filler can be melted, such that only thefiller is melted;

a step of cutting or punching the porous sintered body into apredetermined size, to form a porous cathode substrate;

a step subjecting the porous cathode substrate to tumbling processing,thereby to remove burrs and contaminations;

a step of removing the filler from the porous cathode substratesubjected to the tumbling processing; and

a step of impregnating the porous cathode substrate from which thefiller has been removed, with an electron emission substance.

Fourthly, the present invention provides a method of manufacturing animpregnated-type cathode substrate according to the first aspect of theinvention, characterized by comprising:

a step of forming a sintered body made of high melting point metal toform a large particle diameter low porosity region;

a step of preparing paste containing high melting point metal powderhaving an average particle diameter smaller than that of the largeparticle diameter low porosity region and at least one kind of fillerselected from a group of metal and synthetic resin having a meltingpoint of 1200° C. or less;

a step of applying the paste to an electron emission surface side of theporous sintered body made of high melting point metal to form the largeparticle diameter low porosity region;

a step of heating the porous sintered body made of high melting pointmetal of the large particle diameter low porosity region applied withthe paste, to a temperature at which the filler can be melted, such thata small particle diameter high porosity region having an averageparticle diameter smaller than that of the large particle diameter lowporosity region and a porosity higher than that of the large particlediameter low porosity region is formed, thereby to obtain a porouscathode pellet;

a step of cutting or punching the porous sintered body into apredetermined size, to form a porous cathode substrate;

a step of subjecting the porous cathode substrate to tumblingprocessing, to remove burrs and contaminations;

a step of removing the filler from the porous cathode substratesubjected to the tumbling processing; and

a step of impregnating the porous cathode substrate with an electronemission substance.

Fifthly, the present invention provides an impregnated-type cathodeassembly characterized by including an impregnated-type cathodesubstrate according to the first aspect of the invention.

Sixthly, the present invention provides an electron gun assemblycharacterized by comprising an electron gun provided with animpregnated-type cathode assembly including an impregnated-type cathodesubstrate according to the first aspect of the invention.

Seventhly, the present invention provides an electron tube comprising anelectron gun assembly using an electron gun provided with animpregnated-type cathode assembly including an impregnated-type cathodesubstrate according to the first aspect of the invention.

Since the impregnated-type cathode assembly according to the presentinvention uses an improved cathode substrate, the assembly attains asufficient ion-impact resistance under condition of a high voltage and ahigh frequency, thus achieving an excellent electron emissioncharacteristic.

In addition, since a layer made of a particular substance is formed onan electron emission surface of the impregnated-type cathode, theoperation ability at a low temperature is much improved.

Further, since an impregnated-type cathode having a surface and poreportions of an excellent condition is obtained by using themanufacturing method according to the present invention, it is possibleto provide an impregnated-type cathode assembly which has a sufficiention-impact resistance and an excellent electron emission characteristic.

Furthermore, by using an impregnated-type cathode assembly according tothe present invention, it is possible to obtain an electron gun assemblyand an electron tube which can operate excellently under condition of ahigh voltage and a high frequency.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-section for explaining an example of anelectron gun assembly for a cathode ray tube, according to the presentinvention.

FIG. 2 is a schematic cross-section for explaining a main part of anexample of an electron gun assembly for a klystron tube, according tothe present invention.

FIG. 3 is a schematic cross-section for explaining an example of anelectron tube for a cathode ray tube, according to the presentinvention.

FIG. 4 is a schematic cross-section for explaining a main part of anexample of an electron tube for a klystron tube, according to thepresent invention.

FIG. 5 is a schematic cross-section for explaining an example of anelectron tube for a traveling wave tube, according to the presentinvention.

FIG. 6 is a schematic cross-section for explaining an example of anelectron tube for a gyrotron tube, according to the present invention.

FIG. 7 is a partially cut schematic view showing a first example of animpregnated-type cathode assembly, according to the present invention.

FIG. 8 is a model view showing a structure of the impregnated-typecathode of FIG. 7.

FIG. 9 is a graph showing the electron emission characteristic of theimpregnated-type cathode assembly of FIG. 7.

FIG. 10 is a schematic view showing a structure of a cathode assemblyused in a second example.

FIG. 11 is a model view showing a structure of a cathode assembly usedin a third example.

FIG. 12 is a graph showing the electron emission characteristicaccording to a fifth example.

FIG. 13 is a model view showing a structure of a cathode assembly usedin a sixth example.

FIG. 14 is a graph showing the electron emission characteristicaccording to the sixth example.

FIG. 15 is a view showing steps of manufacturing a cathode substrateused in the present invention.

FIG. 16 is a view showing steps of manufacturing a cathode substrateused in the present invention.

FIG. 17 is a view for explaining steps of manufacturing a cathodesubstrate used in the present invention.

FIG. 18 is a view for explaining steps of manufacturing a cathodesubstrate used in the present invention.

FIG. 19 is a view for explaining steps of manufacturing a cathodesubstrate used in the present invention.

FIG. 20 is a view for explaining steps of manufacturing a cathodesubstrate used in the present invention.

FIG. 21 is a view for explaining steps of manufacturing a cathodesubstrate used in the present invention.

FIG. 22 is a model view showing a structure of a cathode substrateaccording to a seventh example.

FIG. 23 is a model view showing a structure of a cathode substrateaccording to a seventh example.

FIG. 24 is a view for explaining other steps of manufacturing a cathodeassembly used in the present invention.

FIG. 25 is a view for explaining other steps of manufacturing a cathodeassembly used in the present invention.

BEST MODE OF CARRYING OUT THE INVENTION

The present inventors attempted to raise the formation speed of andipole layer on an electron emission surface of an impregnated-typecathode assembly, to be higher than the speed at which the dipole layeris broken or scattered by an ion impact.

An electron emission substance impregnated in a porous cathode substrateis diffused along the surface of metal particles in the substrate fromthe inside of the metal substrate to the electron emission surface, andforms an dipole layer on the electron emission surface.

To shorten the time required until the electron emission substance isdiffused and forms an dipole layer, the diffusion distance may beshortened. As a method of shortening the diffusion distance, there is aneffective method of reducing the particle diameter of the metal of thesubstrate. For example, the particle diameter of W which is metalforming the substrate is generally 3 to 5 μm. The W particles aresintered and a large number of porous portions each having a size of 0.3μm are formed between particles. An electron emission substance isdiffused through these porous portions, and reaches the emissionsurface, thereby forming an dipole layer. If the dipole layer is brokenby an ion impact, a new electron emission substance must be diffusedthrough the porous and supplied to the entire emission surface. In thiscase, if the length of the porous portions through which the electronemission substance passes is short, the diffusion is accelerated, and anew electron emission substance is immediately compensated for, so thata sufficient electron emission characteristic is obtained and theemission is recovered.

The present invention has been made on the basis of the theory asdescribed above, and the first aspect of the invention provides animpregnated-type cathode substrate which contains a large particlediameter low porosity region, and a small particle diameter highporosity region which is provided in the electron emission surface sideof the large particle diameter low porosity region which has a smalleraverage particle diameter than the of the large particle diameter lowporosity region and has a higher porosity than the large particlediameter low porosity region, with said cathode substrate beingimpregnated with an electron emission substance.

More specifically, the impregnated-type cathode substrate according tothe first aspect of the invention contains at least a two-layeredstructure substantially consisting of a first region formed of sinteredparticles of a first average particles diameter and having a firstporosity, and a second region provided at a part of an electron emissionsurface of the first region and having a second average particlediameter smaller than the first average particle diameter and a secondporosity higher than the first porosity. Note that the first region iscalled a large particle diameter low porosity region, and the secondregion is called a small particle diameter low porosity region.

A porous cathode substrate used in the present invention contains, forexample, a sintered body obtained by sintering powder of high meltingpoint metal, e.g., W, molybdenum (Mo), rhenium (Re), or the like.

The term of "average particle diameter" is an average particle diameterof particles forming the sintered body as obtained above.

The entire porous cathode assembly may be impregnated with an electronemission substance, or regions of the assembly except for a partthereof, e.g., except for the vicinity of the electron emission surface,may be impregnated with the electron emission substance.

According to a first preferred embodiment of the first aspect of theinvention, the large particle diameter low porosity region preferablyhas an average particle diameter of 2 to 10 μm and has a porosity of 15to 25%.

More specifically, the impregnated-type cathode substrate according tothe first preferred embodiment of the first aspect of the inventionincludes at least a two-layered structure substantially consisting of alarge particle diameter low porosity region which is formed of sinteredparticles having an average particle diameter of 2 to 10 μm and has aporosity of 15 to 25%, and a small particle diameter high porosityregion which is provided at at least a part of the electron emissionsurface and has a smaller average particle diameter than the averageparticle diameter of the large particles diameter low porosity regionand a higher porosity than the porosity of the large particle diameterlow porosity region.

According to a second preferred embodiment of the first aspect of theinvention, the small particle diameter high porosity region preferablyhas an average particle diameter which is equal to or larger than 0.1 μmand is smaller than 2.0 μm, and has a porosity which is 25% to 40%.

More specifically, the impregnated-type cathode substrate according tothe second preferred embodiment of the first aspect of the inventioncomprises a two-layered structure substantially consisting of a largeparticle diameter low porosity region and a small particle diameter highporosity region which is provided at at least a part of the electronemission surface of the large particle diameter low porosity region andwhich is formed of a sintered body made of particles having an averageparticle diameter which is equal to or larger than 0.1 μm and is smallerthan 2 μm, and which has a porosity of 25 to 40%.

According to a third embodiment of the first aspect of the invention,the small particle diameter high porosity region preferably has athickness of 30 μm or less.

More specifically, the impregnated-type cathode substrate according tothe third preferred embodiment of the first aspect of the inventionincludes at least a two-layered structure substantially consisting of alarge particle diameter low porosity region and a small particlediameter high porosity region which is provided at at least a part ofthe electron emission surface of the large particle diameter lowporosity region and which has a thickness of 30 μm or less.

According to a fourth preferred embodiment of the first aspect of theinvention, the small particle diameter high porosity region ispreferably provided linearly or scattered in the electron emissionsurface side of the large particle diameter low porosity region.

More specifically, the impregnated-type cathode substrate according tothe fourth preferred embodiment of the first aspect of the inventionincludes a structure substantially consisting of a large particlediameter low porosity region and a small particle diameter high porosityregion which is provided linearly or scattered in the electron emissionsurface side.

According to a fifth preferred embodiment of the first invention, theaverage particle diameter and the porosity change in stages from thelarge particle diameter low porosity region to the small particlediameter high porosity region.

More specifically, the impregnated-type cathode substrate according tothe fifth preferred embodiment of the first invention substantially hasa structure in which the average particle diameter decreases in thethickness direction toward the electron emission surface side and inwhich the porosity increases toward the electron emission surface side.

According to a sixth preferred embodiment of the first aspect of theinvention, at least one layer containing at least one kind of elementselected from a group of iridium (Ir), osmium (Os), rhenium (Re),ruthenium (Ru), rhodium (Rh), and scandium (Sc) is further formed on theelectron emission surface.

More specifically, the impregnated-type cathode substrate according tothe sixth embodiment of the first aspect of the invention includes alayered structure consisting of at least three layers of a largeparticle diameter low porosity region, a small particle diameter highporosity region provided in the electron emission side, and at least onelayer including at least one kind of element selected from a group ofiridium, osmium, rhenium, ruthenium, rhodium, and scandium.

In the first aspect of the invention, the entire porous cathodesubstrate may be impregnated with an electron emission substance, orregion of the substrate except for a part thereof, e.g., except for thevicinity of the electron emission surface, may be impregnated with anelectron emission substance. Otherwise, only the large particle diameterlow porosity region may be impregnated with an electron emissionsubstance.

The second aspect of the invention provides a method of manufacturing animpregnated-type cathode, as a preferred method of manufacturing animpregnated- type cathode substrate according to the first aspect of theinvention, said method comprising:

(1) a step of forming a porous sintered body having a large particlediameter and a low porosity;

(2) a step of obtaining a porous cathode pellet by forming a smallparticle diameter high porosity region in the electron emission surfaceside of the porous sintered body, said small particle diameter highporosity region having a smaller average particle diameter than theaverage particle diameter and a higher porosity than the porosity of thelarge particle diameter low porosity region;

(3) a step of cutting or punching the porous pellet, to form a porouscathode substrate; and

(4) a step of impregnating the porous cathode substrate with an electronemission substance.

The small particle diameter high porosity region is preferably formed bya method selected from a group of a printing method, a spin-coatingmethod, a spray method, an electrocoating method, and an elution method.

The third aspect of the invention relates to an improved version of themethod according to the second aspect of the invention and provides amethod of manufacturing an impregnated-type cathode substrate,characterized by comprising:

(1) a step of forming a porous sintered body having a large particlediameter and a low porosity;

(2) a step of obtaining a porous cathode pellet by forming a smallparticle diameter high porosity region in the electron emission surfaceside of the porous sintered body, said small particle diameter highporosity region having a smaller average particle diameter than theaverage particle diameter of the large particle diameter low porosityregion and a higher porosity than the porosity of the large particlediameter low porosity region;

(3) a step of providing a filler selected from a group of metal andsynthetic resin having a melting point of 1200° C. or less, in anelectron emission surface side of the porous cathode pellet;

(4) a step of heating a formed resultant including the filler, at atemperature at which the filler can be melted, such that only the filleris melted;

(5) a step of cutting or punching the porous sintered body in apredetermined size, to form a porous cathode substrate, and ofsubjecting the porous cathode substrate to tumbling processing, therebyto remove burrs and contaminations;

(6) a step of removing the filler from the porous cathode substratesubjected to the tumbling processing; and

(7) a step of impregnating the porous cathode substrate from which thefiller has been removed, with an electron emission substance.

Note that the porous cathode pellet means a porous cathode substratebefore being subjected to processing of cutting or punching the baseinto a porous cathode substrate having a predetermined shape.

According to the fourth aspect of the invention, there is provided amethod of manufacturing an impregnated-type cathode substrate,characterized by comprising:

(1) a step of forming a sintered body made of high melting point metalas a large particle diameter low porosity region;

(2) a step of applying paste containing high melting point metalparticle having a smaller average particle diameter than an averageparticle diameter of the large particle diameter low porosity region andat least one kind of filler selected from a group of metal and syntheticresin having a melting point of 1200° C. or less, to an electronemission surface side of the porous sintered body, and of performingbaking at a temperature at which the filler can be melted, thereby toform a porous sintered body as a small particle diameter high porosityregion and to melt the filler in the porous sintered body;

(3) a step of cutting or punching the porous sintered body in apredetermined size, to form a porous cathode substrate;

(4) a step of subjecting the porous cathode substrate to tumblingprocessing, to remove burrs and contaminations;

(5) a step of removing the filler from the porous cathode substratesubjected to the tumbling processing; and

(6) a step of impregnating the porous cathode substrate with an electronemission substance.

Further, it is possible to form an impregnated-type cathode assemblywith use of a porous cathode substrate thus obtained. Also, it ispossible to form an electron tube with use of the impregnated-typecathode assembly.

The fifth invention provides a porous cathode assembly which uses theporous cathode substrate according to the first aspect of the inventionand which is used for, for example, a porous cathode assembly for acathode ray tube, a porous cathode assembly for a klystron tube, aporous cathode assembly for a traveling wave tube, and a porous cathodeassembly for a gyrotron tube.

More specifically, the impregnated-type cathode assembly of the fifthinvention is a porous cathode assembly comprising a porous cathodesubstrate which consists of a sintered body made of high melting pointmetal particle and which is impregnated with an electron emissionsubstance, a support member for supporting the porous cathode substrate,and a heater provided in the support member, wherein the porous cathodesubstrate substantially consists of a large particle diameter lowporosity region made of sintered particle and having a first porosity,and a small particle diameter high porosity region which is provided atleast a part of an electron emission surface of the large particlediameter low porosity region and which has a second average particlediameter smaller than the first average particle diameter and a secondporosity higher than the first porosity.

An impregnated-type cathode assembly according to a first embodiment ofthe fifth invention is a cathode assembly comprising a porous cathodesubstrate which is impregnated with an electron emission substance andis formed of a sintered body of high melting point metal powder, asupport member for supporting the porous cathode substrate, and a heaterprovided in the support member, wherein the porous cathode substrate hasat least a two-layered structure substantially consists of a largeparticle diameter low porosity region which is made of sinteredparticles having an average diameter of 2 to 10 μm and which has aporosity of 15 to 25%, and a small particle diameter high porosityregion which is provided at least a part of an electron emission surfaceand has a porosity higher than the porosity of the large particlediameter low porosity region.

An impregnated-type cathode assembly according to a second embodiment ofthe fifth invention is a cathode assembly comprising a cathode substratewhich is impregnated with an electron emission substance and is formedof a porous sintered body of high melting point metal particle, asupport member for supporting the porous cathode substrate, and a heaterprovided in the support member, wherein the porous cathode substrate hasat least a two-layered structure substantially consists of a largeparticle diameter low porosity region and a small particle diameter highporosity region which is provided at least a part of an electronemission surface of the large particle diameter low porosity region andwhich contains a sintered body made of particles having an averageparticle diameter which is 0.1 μm or more and is less than 2.0 μm, saidsmall particle diameter high porosity region having a porosity of 25 to40%.

An impregnated-type cathode assembly according to a third embodiment ofthe fifth invention is a cathode assembly comprising a porous cathodesubstrate having a two-layered structure substantially consisting of alarge particle diameter low porosity region and a small particlediameter high porosity region, a support member for supporting thecathode substrate, and a heater provided in the support member, saidsmall particle diameter high porosity region being provided at least apart of an electron emission surface of the large particle diameter lowporosity region and having a thickness of 30 μm or less.

An impregnated-type cathode assembly according to a fourth embodiment ofthe fifth invention is a cathode assembly comprising a porous cathodesubstrate having a two-layered structure substantially consisting of alarge particle diameter low porosity region and a small particlediameter high porosity region, a support member for supporting thecathode substrate, and a heater provided in the support member, saidsmall particle diameter high porosity region being provided linearly orscattered in an electron emission surface side of the large particlediameter low porosity region.

An impregnated-type cathode assembly according to a fifth embodiment ofthe fifth invention is a cathode assembly comprising a porous cathodesubstrate, a support member for supporting the cathode substrate, and aheater provided in the support member, said porous cathode substratesubstantially having a layered structure substantially consisting ofthree or more layers of a large particle diameter low porosity region, asmall particle diameter high porosity region provided in an electronemission surface side, and at least one layer containing at least onekind of element selected from a group of iridium, osmium, rhenium,ruthenium, rhodium, and scandium.

In case where the cathode assembly according to the fifth invention isused for a cathode ray tube, the cathode assembly includes, for example,a cylindrical cathode sleeve, an impregnated-type cathode substratefixing member fixed to an inner surface of an end portion of the cathodesleeve, an impregnated-type cathode substrate according to the firstembodiment fixed to the impregnated-type cathode substrate fixingmember, a cylindrical holder provided coaxially outside the cathode soas to surround the cathode sleeve, a plurality of straps each having anend portion fixed to the outside of the cathode sleeve and another endportion fixed to the inside of the cylindrical holder, and a heaterprovided inside the cathode sleeve.

In case where the cathode assembly according to the fifth invention isused for a klystron tube, the cathode assembly includes, for example, animpregnated-type cathode substrate, a support cylinder for supportingthe impregnated-type substrate, a heater included in the supportcylinder and embedded in an insulating material.

A sixth aspect of the invention uses a porous cathode substrateaccording to the first aspect of the invention to provide an electrongun assembly for a cathode ray tube, a klystron tube, a traveling wavetube, and a gyrotron tube.

In case where the electron gun assembly according to the sixth aspect ofthe invention is an electron gun assembly for a cathode ray tube, theassembly includes, for example, an impregnated-type cathode assemblyaccording to the fifth invention, a plurality of grid electrodescoaxially provided in an electron emission surface side of theimpregnated-type cathode assembly, an electron gun having a convergenceelectrode coaxially provided in front of the plurality of gridelectrodes, and a resistor as a voltage divider connected to theelectron gun.

FIG. 1 is a schematic cross-section showing a color picture tubeincorporating a resistor included in an electron tube, as an example ofthe electron gun assembly for a cathode ray tube according to the sixthaspect of the invention.

In FIG. 1, the reference 61 denotes a vacuum container, and an electrongun assembly A is provided inside a neck portion 61a formed in thevacuum container 61. In the electron gun assembly A, a first gridelectrode G1, a second grid electrode G2, a third grid electrode G3, afourth grid electrode G4, a fifth grid electrode G5, a sixth gridelectrode G6, a seventh grid electrode G7, and an eighth grid electrodeG8 are coaxially formed in this order, commonly with respect to threecathodes. A convergence electrode 62 is provided in the rear stagebehind the after the grid electrode G8.

The grid electrodes G1, G2, G3, G4, G5, G6, G7 and G8 maintain apredetermined positional relationship, and are mechanically held by beadglass 3. In addition, the third grid electrode G3 and the fifth gridelectrode G5 are electrically connected with each other by a lead line64. The convergence electrode 62 is connected with the eighth gridelectrode by welding.

In this electron gun assembly A, a resistor 65 incorporated in anelectron tube is provided. This resistor 65 comprises an insulatingboard 65A. A resistor layer (not shown) and an electrode layer connectedto this resistor layer are formed on this insulating board 65A. Theinsulating board 65A of this resistor 65 is provided with terminals 66a,66b, and 66c for drawing high voltage electrodes to be connected to theelectrode layer, and the terminals 66a, 66b, and 66c are respectivelyconnected to the seventh grid electrode G7, sixth grid electrode G6, andfifth grid electrode G5. A terminal 67 provided on the insulating board65A of the resistor 65 and connected to the electrode layer is connectedto the convergence electrode 62, and a drawing terminal 68 of the earthside which is provided on the insulating board 65A and connected to theelectrode layer is connected to the earth electrode pin 69.

Meanwhile, a graphite conductive film 70 extending to the inner wall ofthe neck portion 61a is coated on the inner wall of a funnel portion 61bof the vacuum container 61, and the graphite conductive film 70 issupplied with an anode voltage through a high voltage supply button(which is an anode button not shown).

Further, the convergence electrode 62 is provided with a conductivespring 79, and the conductive spring 79 is brought into contact with thegraphite conductive film 70, so that an anode voltage is supplied to theeight grid electrode G8 through the convergence electrode 62 and to theconvergence terminal 67 of the resister 65 incorporated in the electrontube, and divisional voltages generated at the electrodes 66a, 66b, and66c of a high voltage are respectively supplied to the seventh gridelectrode G7, sixth grid electrode G6, and fifth grid electrode G5.

In case where the electron gun assembly according to the sixth aspect ofthe invention is an electron gun assembly for a klystron tube, theassembly includes an impregnated-type cathode assembly according to thefifth invention, a cathode portion incorporating the impregnated-typecathode assembly, and an anode portion coaxially provided on theelectron emission surface of the impregnated-type cathode assembly.

FIG. 2 is a schematic cross-section for explaining a main part of anexample of an electron gun assembly for a klystron tube according to thesixth aspect of the invention.

As shown in FIG. 2, in the main part of the example of an electron gunassembly for a klystron tube, a cathode portion 181 where a cathodeassembly 81 is provided and an insulating portion 93 are sealed by awelding flange 180 formed of a thin metal ring engaged and tapered alongthe axial direction, and by an arc welding sealing portion 184 at thetop end of the cathode portion 181. In addition, the insulating portion93 and the anode portion 95 are air-tightly sealed by a welding flange182 formed of a thin metal ring engaged and tapered along the axialdirection and by a top arc welding sealing portion of the portion 183.In order to assembly the electron gun assembly while defining thedistances of electrodes to the anode portion 95, the insulating portion93 and the anode portion 95 are engaged with each other finally, and areair-tightly sealed by the welding sealing portion 98.

In general cases, a difference in electrode distances from designeddimensions can be cited as a drawback of an electron gun assembly whichmay seriously affect the operation of a klystron tube. The difference ismainly caused by precision of components and precision of assembly.Therefore, the electrode distances are adjusted in the following manner.Specifically, as for a difference in the axial direction, an appropriateconductive spacer is inserted between a stem plate 84 of the cathodeportion and a stem end plate 86, and is fixed by a screw 85, or a spaceris inserted between a back-up ceramics ring 92 and a welding flange 180or 183. As for a difference in the radial direction, an axial adjustmentwith respect to a Wehnelt member 82 and a welding flange 180 is carriedout with use of a rotation base tool, and thereafter, the cathodeportion 83 is fixed by a screw 85. As for the insulating portion 93,brazing is carried out with use of an appropriate assembly tool so thatthe welding flanges 181 and 182 obtain a concentricity.

In addition, the seventh aspect of the invention uses animpregnated-type cathode substrate according to the first aspect of theinvention to provide an electron tube used for, for example, a cathoderay tube, a klystron tube, a traveling tube, and a gyrotron tube.

In case where the electron tube according to the seventh aspect of theinvention is used for a cathode ray tube, the electron tube includes,for example, a vacuum outer envelope having a face portion, afluorescent layer provided on an inner surface of the face portion, anelectron gun assembly according to the sixth aspect of the invention andprovided at a position opposite to the face portion of the vacuum outerenvelope, and a shadow mask provided between the fluorescent layer andthe electron gun assembly.

FIG. 3 is a schematic cross-section for explaining an example of anelectron tube for a cathode ray tube according to the present invention.

As shown in FIG. 3, the electron tube for this cathode ray tube has anouter envelope consisting of a rectangular panel 31, a funnel 32, and aneck 33. On the inner surface of the panel 31, a fluorescent layer 34which emits light in red, green, and blue is provided like stripes. Inthe neck 33, an in-line type electron gun 36 which injects electronbeams 35 corresponding colors of red, green, and blue is provided, andthe electron gun 36 is constituted by arranging electron gun assembly asshown in FIG. 1 in line. At a position adjacent to and opposite to thephosphor member 34, a shadow mask 7 having a number of fine openingholes is supported by and fixed to a mask frame 38. An image isreproduced by deflecting electron beams by a deflecting device 38,thereby to perform scanning.

In case where the electron tube is used for a klystron tube, theelectron tube includes an electron gun assembly according to the sixthaspect of the invention, a high frequency acting portion and a collectorportion in which a plurality of resonance cavities arranged coaxially inan electron emission surface side of the electron gun assembly areconnected by a drift tube, and a magnetic field generator deviceprovided in an outer peripheral portion of the high frequency actingportion.

FIG. 4 is a schematic cross-section for explaining a main part of anexample of an electron tube for a klystron tube according to the presentinvention.

As shown in FIG. 4, in the main part of the electron tube for a klystrontube, the reference 191 denotes an electron gun portion, and thereference 192 denotes a cathode assembly. A high frequency actingportion 195 in which a plurality of resonance cavities 193 are connectedby a drift tube 194 and a collector portion 196 are connected in thisorder with an electron gun portion having a structure as shown in FIG.2. Further, a magnetic field generator device, e.g., an electromagnetcoil 197 is provided outside the high frequency acting portion 195. Notethat the reference 198 denotes an electron beam. In addition, the outputwaveguide portion is omitted from the figure.

In case where the electron gun according to the seventh aspect of theinvention is used for a traveling wave tube, the electron tube includesan electron gun assembly using an impregnated-type cathode assemblyaccording to the present invention, a slow-wave circuit for amplifying asignal provided coaxially in an electron emission surface side of theimpregnated-type cathode assembly, and a collector portion for capturingan electron beam.

FIG. 5 is a schematic cross-section for explaining an example of anelectron tube for a traveling wave tube according to the presentinvention.

As shown in FIG. 5, this traveling wave tube comprises an electron gun171, a slow-wave circuit (or high frequency acting portion) 172 foramplifying a signal, and a collector 173 for capturing an electron beam.The slow-wave circuit 172 is constituted such that a helix coil 175 issupported by and fixed to three dielectric support rods 176 in apipe-like vacuum envelope 174, and an input contact plug 177 and anoutput contact plug 178 are projected at both ends of the slow-wavecircuit 172.

In case where electron tube according to the seventh aspect of theinvention is used for a gyrotron, the electron tube includes, forexample, an electron gun assembly using an impregnated-type cathodeassembly according to the present invention, a tapered electron beamcompressing portion which is provided in an electron emission surfaceside of the impregnated-type cathode assembly and whose diametergradually decreases, a cavity resonance portion arranged to becontinuous to the tapered electron beam compressing portion, a taperedelectromagnetic waveguide portion which is arranged to be continuous tothe cavity resonance portion and whose diameter gradually increases, acollector portion for capturing an electron beam, and a magnetic fieldgenerator device provided at an outer peripheral portion of the cavityresonance portion.

FIG. 6 is a schematic cross-section for explaining an example of anelectron tube for a gyrotron tube according to the present invention.

In FIG. 6, the reference 230 denotes a body of a gyrotron tube, and thereference 231 denotes a hollow electron gun portion which is assembledwith use of an impregnated-type cathode assembly and generates anelectron beam. The reference 232 denotes a tapered electron beamcompressing portion which is provided in the down stream side of theelectron beam and whose diameter gradually decreases, and the reference233 denotes a tapered electromagnetic wave guide portion which isprovided in the down stream side of the compressing portion and whosediameter gradually decreases. The reference 235 denotes a collectorportion which is provided behind the wave guide portion and captures anelectron beam after interaction is performed. The reference 236 denotesan output window which is provided in the down stream side of thecollector portion and has a ceramics air-tight window. The reference 237denotes a waveguide tube connection flange, and the reference 239denotes a solenoid valve of a magnetic field generator device.

The first aspect of the invention will now be explained below.

In the first aspect of the invention, a porous region having a smallparticle diameter and a high porosity and a porous region having a largeparticle diameter and a low porosity are provided in this order from atleast the electron emission side of the impregnated-type cathodeassembly.

In the large particle diameter low porosity region, supply of animpregnated electron emission substance can be maintained constantduring heating.

In addition, since the small particle diameter high porosity region isprovided on the large particle diameter low porosity region, distancesbetween particles forming the cathode substrate are short within thesmall particle diameter high porosity region in the electron emissionsurface side, so that the diffusion distance of the electron emissionsubstance is shortened. Therefore, the electron emission substancecovers the electron emission surface more rapidly and uniformly, so thatsufficient supply of an electron emission substance and a sufficientcovering rate concerning the electron emission surface can be achieved.As the covering rate is improved, the ion-impact resistance becomes moreexcellent. In this manner, the aging time of an impregnated-type cathodeassembly which can be operated at a high voltage can be shortened. Inaddition, even if an electron emission substance whose diffusion speedis low is contained, deterioration in electron emission characteristicof the impregnated-type cathode assembly due to an ion impact can beprevented.

The term of "porosity" used in the present invention is a rate of aspace existing in an object (solid) of a constant volume, and isexpressed by the following relation (1).

    P1-W/Vd                                                    (1)

In this relation, w is a weight (g) of an object to be measured, and Vis a volume (cm³) of an object to be measured, is a density of an objectto be measured (e.g., 19.3 g/cm³ when the object is tungsten), and P isa porosity (%). However, a small particle high porosity region requiredin the present invention is preferably a layer state. Further, thislayer preferably has a thickness of 30 μm or less. Therefore, it issubstantially impossible to actually measure the values of w and V, sothat the porosity cannot be calculated. To control actually theporosity, the porosity can be measured in the following method.

At first, in case of a cathode substrate after impregnation, all theelectron emission substance in pores is removed, and thereafter, coloredresin is melted and impregnated in these pores. Thereafter, polishing isperformed by a metal polisher or the like, to form a verticalcross-section on the cathode surface. When the size of the cathodesubstrate is large, the cathode may be previously cut to prepare a roughcross-section. After a smooth cross-section is attained, the image ofthe cross-section is photographed by an optical microscope or anelectronic microscope. The image of this cross-section is subjected toimage processing, for example, by CV-100 available from KEYENCE, toobtain the area S_(base) of a portion where the high melting point metalappears and the area S_(base) of a portion where colored resin appears.Then, P=S_(pore) /(S_(pore) +S_(base))×100 (%) can be used as aporosity. Here, the boundary between the region S_(pore) and the outerregion of the cathode substrate is a line segment connecting points ofhigh melting point metal particles which exist in the outermostcircumference of the cathode substrate and project to the outermostportion from the cathode substrate. Although calculation of the areaS_(base) and the area S_(pore) is preferably performed with respect tothe entire surface of the cathode substrate, it is practically difficultto carry out this calculation. Therefore, at least five points arearbitrarily selected on the cross-section of the cathode substrate, andthe area S_(base) and the area S_(pore) are obtained with respect to anarea of 1000 μm² or more in the vicinity of each of the points. Thevalue of P calculated from the average values can be used as a porosity.

In the first preferred embodiment of the first aspect of the invention,closed pores cannot be neglected as the sintering proceeds duringmanufacturing steps, so that there are no advantages for impregnation ofan electron emission substance even though a certain porosity can beobtained, if the particle diameter of the large particle diameter lowporosity region is 2 μm or less. If the particle diameter exceeds 10 μm,a desired porosity cannot be obtained, so that supply of an electronemission substance to the small particle diameter high porosity regionis insufficient, and there is a tendency that the sintering temperaturebecome extremely high to obtain a desired porosity. Also, there is atendency that industrial manufacture is difficult. A preferable averageparticle diameter of the large particle diameter low porosity region is2 to 7 μm, and a more preferable average particle diameter is 2 to 5 μm.If the porosity is 15% or less, there is a tendency that supply of anelectron emission substance to the small particle diameter high porosityregion is insufficient. If the porosity exceeds 25%, a necessarystrength cannot be obtained and consumption of an electron emissionsubstance is increased so that the life-time is shortened. A preferableporosity of the large particle diameter low porosity region is 15 to22%, and a more preferable porosity is 17 to 21%.

In the second preferred embodiment of the first aspect of the invention,if the average particle diameter of the small particle diameter highporosity region is 0.1 μm or less, the particle diameter is so smallthat the cathode substrate is easily cracked and the strength islowered. Further, if the particle diameter of high melting point metalas raw material is too small, secondary or tertiary particles are formedduring sintering so that the sintering easily prosecutes and a desiredparticle diameter cannot be obtained. In this case, there is a tendencythat the density is increased and a desired porosity cannot be obtained.

In addition, if the particle diameter is 2 μm or more, the diffusiondistance of the electron emission substance is large, so that it takes along time to supply the electron emission surface with a sufficientelectron emission substance. Further, if the diffusion distance islarge, it is difficult to obtain uniform diffusion on the electronemission surface. Hence, it can be found that the covering rate of theelectron emission substance on the electron emission surface decreases.As described above, a sufficient ion-impact resistance cannot beobtained if the covering rate decreases.

A more preferable average particle diameter of the small particlediameter high porosity region of the porous cathode substrate is 0.8 to1.5 μm.

If the porosity is 25% or less where the average particle diameter ofthe small diameter high porosity region is within a range of 0.1 μm to2.0 μm, there is a tendency that an electron emission substance cannotbe sufficiently supplied to the electron emission surface and thecovering rate of the electron emission substance on the electronemission surface decreases. If the covering rate decreases, a sufficiention-impact resistance cannot be obtained.

If the porosity is high and exceeds 40% where the average particlediameter of the cathode substrate is within a range of 0.1 μm to 2.0 μm,the mechanical strength of the cathode substrate tends to decrease. Amore preferable porosity of the small particle diameter high porosityregion is 25 to 35%.

In case of an impregnated-type cathode substrate having a layeredstructure comprising of at least two layers as shown in the thirdpreferred embodiment of the first aspect of the invention, the layerthickness of the small particle diameter high porosity region providedin the electron emission surface side of the large particle diameter lowporosity region is preferably 30 μm. This layer thickness is morepreferably 3 to 30 μm, and is most preferably 3 to 20 μm.

As shown in the second aspect of the invention, the impregnated-typecathode assembly having at least two-layered structure can bemanufactured in the following manner.

At first, a normal method is used to form a porous sintered body as alarge particle diameter low porosity region which has an averageparticle diameter of 2 to 10 μm and a porosity of 15 to 20%.

In the next, high melting point metal of powder W having an averageparticle diameter smaller than the average particle diameter of a poroussintered body as a large particle diameter low porosity region isprepared in form of paste together with an organic solvent, on theelectron emission surface of the porous sintered body, and is applied bya screen printing method, to have a desired thickness. Thereafter, thepaste is dried and is subjected to sintering within a temperature rangeof 1700 to 2200° C., in a vacuum atmosphere or a reducing atmosphereusing hydrogen (H₂). Thus, a small particle diameter high porosityregion is formed on the large particle diameter low porosity region. Inthis case, the density of paste, printing conditions, and the sinteringtime may be appropriately set such that the particles forming thesintered body have a desired average particles diameter and a desiredporosity.

In addition, as another structure of a cathode substrate according tothe first aspect of the invention, a structure can be cited in which aplurality of small particle diameter high porosity regions are scatteredat least in the electron emission side of a matrix formed of a largeparticle diameter low porosity region, as shown in the fourth preferredembodiment. For example, a concave portion exists like a groove or ahole in the electron emission surface of the large particle diameter lowporosity region, and the small particle diameter high porosity regionexists in the concave portion. To form a cathode assembly having suchstructure, a groove-like or hole-like concave portion is formed in theelectron emission surface side of the porous sintered body as the largeparticle diameter low porosity region, by mechanical processing or thelike, and paste is filled in the concave portion. The paste is subjectedto sintering to form a small particle diameter high porosity region.

Further, as another modification of the structure of the cathodesubstrate, a structure can be cited in which the porosity graduallyincreases in the thickness direction toward the electron emissionsurface, while the particle diameter gradually decreases in the samedirection, as shown in the fifth preferred embodiment of the firstinvention.

Formation of the small particle diameter high porosity region is notlimited to the printing method described above, but any method includinga spin coating method, a spray method, an electrocoating method, and anelution method can be adopted as long as a porous layer can be obtainedby such a method. In case where the elution method is adopted, asintering step can be omitted.

As for the cathode substrate of a cathode assembly having the structureas described above, for example, an electron emission substance made ofa mixture substance which has a mole ratio of BaO:CaO:Al₂ O₃ is 4:1:1 ismelted and impregnated in a reducing atmosphere of hydrogen H₂.

Further, the sixth preferred embodiment of the first aspect of theinvention will be explained below.

The at least one kind of element selected from a group of iridium (Ir),osmium (Os), rhenium (Re), ruthenium (Ru), rhodium (Rh), and scandium(Sc) which is used in the sixth preferred embodiment of the first aspectof the invention can be used in single use, in form of a substancecontaining the selected element, or in combination with another elementor with a substance containing another element.

The combination includes a case where different elements existsindependently from each other and a case where different elements existin form of an alloy or a compound.

According to the sixth preferred embodiment, since a layer containingthose elements is formed, electron emission characteristic can berapidly recovered so that emission and sufficient low temperatureoperation are enabled even when an dipole layer on the electron emissionsurface of the cathode assembly is broken. In addition, since lowtemperature operation is achieved, an amount of an evaporation electronemission substance such as barium or the like can be lowered and thethickness of the cathode assembly can be thin.

Elements which are preferably used in single use are iridium andscandium.

Substances containing preferable elements are scandium oxide (SC₂ O₃)and scandium hydride (ScH₂).

Preferable combinations of elements are alloys of Ir--W, Os--Ru, Sc₂ O₃--W, Sc--W, ScH₂ --W, Sc--Re.

Although Os can be singly used in view of its functions, it is morepreferable to use Os in form of alloy which is less oxidized rather thanin single use, in view of safety of operators, since oxide material ofOs is poisonous.

Sc can be used in combination with at least one kind of element selectedfrom a group of high melting point metal such as hafnium (Hf), rhenium(Re), ruthenium (Ru), and the likes. These kinds of high melting pointmetal serve as a segregator which prevents Sc from oxydization duringoperation of a cathode assembly.

In addition, in the first aspect of the invention, excessive elementemission substances are removed from the surface of the porous cathodesubstrate if necessary, and thereafter, a layer of element components tobe used can be formed by a thin film formation means such as asputtering method or the like.

The third aspect of the invention and the fourth aspect of the inventionwill further be explained below.

The third aspect of the invention and the fourth aspect of the inventionare to improve a step of cutting a cathode substrate having apredetermined form from a porous body, in a manufacturing method of aporous cathode assembly. A cut out cathode substrate has burrs.Therefore, the cathode substrate is subjected to tumbling processing toremove burrs. Normally, tumbling processing is carried out by shaking acut out cathode substrate together with small balls made of alumina andsilica in a container, thereby rubbing the small ball and the cathodesubstrate with each other. In this state, the electron emission surfaceside can be rubbed in the same manner, so that pore portions of theporous body are closed. Since the porous portions are supply paths foran electron emission substance, there issues a problem that impregnationof the electron emission substance is prevented if the pore portions areclosed. In addition, the apparent surface area of the porous bodysurface is increased, resulting in a problem that the diffusion distanceof the electron emission substance on the surface is increased.Particularly, in a cathode substrate having a small grin diameter highporosity region, shortening of the diffusion distance of an electronemission substance and enlargement of supply paths are affected due tothose problems, so that advantageous improvements in the ion-impactresistance characteristic cannot be attained.

In addition, when the surface of a cathode substrate is pealed, anelectron emission substance blows out, thereby causing qualitydeterioration in the electron emission surface. The qualitydeterioration in the electron emission surface cause an influence suchas a deterioration in the emission current density.

According to the third aspect of the invention, a filler selected frommetal and synthetic resin having a melting point of 1200° C. or less isapplied to the surface of the electron emission surface of the porousbody before a cathode substrate is cut and processed, and is subjectedto a heating treatment, to melt the filler in the porous body formingmaterial. As a result, the filler is melted into the porous body throughpore portions in the electron emission surface. In this manner, theinside of the pores and the porous body are reinforced, so that poreportions are not closed even when the electron emission surface isrubbed during tumbling processing.

According to the fourth aspect of the invention, paste containing highmelting point metal and at least one kind of filler selected from agroup of metal and synthetic resin having a melting point of 1200° C. orless is sintered at a temperature at which the filler can be melted, toform a porous body containing high melting point metal as a maincomponent and to melt the filler into the pores of the porous body. As aresult of this, the inside of the pores and the porous body arereinforced, so that the pore portions are not closed even when theelectron emission surface is rubbed during tumbling processing.

In addition, as an example of application of the cathode substrateaccording to the present invention, a mixture layer of fine powder ofhigh melting point metal and scandium oxide can further be formed on theelectron emission surface region of the cathode substrate. As a resultof this, the electron emission characteristic can be rapidly recoveredand emission and sufficient low temperature operation can be enabledagain, even when an electric double layer on the electron emissionsurface of the cathode substrate is broken by an ion impact. Inaddition, since low temperature operation is thus enabled, theevaporation amount of an electron emission substance such as barium orthe like can be reduced to be low, so that the thickness of a cathodesubstrate can be set to be thinner than a conventional case. This alsomeans that the life-time characteristic of a conventional power-savingimpregnated-type cathode can be greatly improved, which would otherwisebe insufficient due to shortage in impregnation amount of an electronemission substance.

Further, it is preferable that an alloy of tungsten and molybdenum or amixture there of can be used as fine powder of high melting point metal.As a result of this, a sintered layer which is sufficiently strong canbe obtained at a low sintering temperature. As synthetic resin, it ispreferable to use methyl methacrylate.

A sintered layer of fine powder thus obtained preferably has an averageparticle diameter of 0.8 to 1.5 μm, and also preferably has a porosityof 20 to 40% and more preferably has a porosity of 25 to 35%.

In the following, the present invention will be specifically explainedwith reference to the drawings.

Embodiment 1

FIG. 7 is a partially cut schematic view showing an example of anelectron tube using the first embodiment of the impregnated-type cathodeassembly according to the present invention. This cathode assembly is animpregnated-type cathode assembly for a klystron tube and is used with ahigh output and a high voltage.

As shown in the figure, this electron tube mainly comprises, forexample, a metal substrate 3 made of porous material W, a supportcylinder 11 made of Mo or the like brazed so as to support the porouscathode substrate 3, and a heater 18 incorporated in the supportcylinder 11. The heater 18 is fixed in such a manner in which the heateris embedded in a potting material and is subjected to sintering. Poreportions of the porous cathode substrate 3 is impregnated with anelectron emission substance whose mole ratio of BaO:CaO:Al₂ O₃ is 4:1:1.A thin film layer of Ir is provided on the electron emission surfaceside of the porous cathode substrate 3, by means of sputtering, and analloy layer of Ir and W not shown is formed by means of alloyingprocessing. In addition, this cathode assembly has a curvature of, forexample, a radius 53 mm for the purpose of focusing.

FIG. 8 is a model view showing a structure of the porous cathodesubstrate 3 of the cathode assembly. The porous cathode substrate 3 hasa two-layered structure consisting of a large particle diameter lowporosity layer 22 and a small particle diameter high porosity layer 23formed thereon, as is shown in FIG. 8. The porous cathode substrate 3having this structure can be formed by a spraying method as will bedescribed below.

At first, for example, a porous W base which is made of particles havingan average particle diameter of about 3 μm and which have a porosity ofabout 17% is prepared as a large particle diameter low porosity layer.This substrate has, for example, a diameter of 70 mm and has an electronemission surface whose curvature radius is 53 mm.

With this porous W base equipped with a mask tool, a mixture of Wparticles, butyl acetate, and methanol is sprayed vertically onto theelectron emission surface of the substrate, by means of a spray gun.

While the spraying distance was set to 10 cm, the air pressure was setto 1.2 kgf/cm², the spraying flow amount was set to 0.35 cc/sec, and thespraying time was set to 5 seconds, a thin film layer having a thicknessof 20 μm was uniformly formed on the electron emission surface having acurvature.

Thereafter, a heat treatment for one hour was carried out for thepurpose of sintering of the thin film layer and adhesion of the thinfilm layer and the substrate metal, in a reducing atmosphere at atemperature of 1700 to 2200° C., e.g., in a hydrogen atmosphere at atemperature of 2000° C.

A small particle diameter high porosity W thin film layer thus obtainedwas not cracked, and has a sufficient strength. The layer had an averageparticle diameter of 0.8 μm, a porosity of 30%, and an uniform thicknessof about 10 μm.

In the next, an electron emission substance of a mixture whose moleratio of BaO:CaO:Al₂ O₃ is 4:1:1 was melted and impregnated in poreportions of the porous substrate 3, by performing heating in anatmosphere of H₂ at a temperature of 1700° C. for about 10 minutes.

A cathode substrate having a two-layered structure thus obtained was setin a klystron electron tube, and was subjected to aging under conditionthat the cathode temperature was 1000° C. b (°C. b is a brightnesstemperature).

FIG. 9 shows the electron emission characteristic after aging wasperformed for 100 hours. This electron emission characteristic shows arelationship between an emission current and the cathode temperaturewherein the emission current is expressed as a rate with respect to anemission current at a cathode temperature of 1100° C. b as 100%. In thisfigure, solid lines 31 and 32 respectively indicate the characteristicsof a conventional impregnated-type cathode assembly and animpregnated-type cathode assembly according to the embodiment 1. As canbe seen from this graph, the impregnated-type cathode assembly indicatedby the solid line 32 according to the first embodiment is superior at alow temperature. Since the diffusion rate is high at a high temperature,any particular superiority of the impregnated-type cathode assembly ofthe present invention cannot be found at a high temperature. However,since the diffusion rate is low at a low temperature, it can be saidthat the impregnated-type cathode assembly of the present invention isapparently superior. Also, from this graph, it is apparent that theaging time can be shortened by using the impregnated-type cathodeassembly according to the present invention.

Embodiment 2

FIG. 10 is a schematic view showing a second example of theimpregnated-type cathode assembly used for another electron tube,according to the present invention. This cathode assembly is a cathodeassembly for a cathode ray tube, and the cathode substrate thereof doesnot substantially have a curvature, unlike the cathode substrate for aklystron tube according to the embodiment 1.

As shown in the figure, the electron tube using the impregnated-typecathode assembly comprises, for example, a cathode sleeve 1, a cup-likefixing member 2 fixed to the inside of an end portion of the cathodesleeve 1 such that the member 2 forms a plane which is substantially thesame as the opening edge of the end portion, a porous cathode substrate3 fixed in the cup-like fixing member 2 and impregnated with an electronemission substance, a cylindrical holder 4 provided coaxially so as tosurround the cathode sleeve 1, a plurality of strip-like straps 5 eachhaving an end portion attached to the outer surface of the other end ofthe cathode sleeve 1 and having another end portion attached to an innerprojecting portion formed at an end portion of the cylindrical holder 4such that the cathode sleeve 1 is coaxially supported inside thecylindrical holder 4, and a shielding cylinder 7 which is attached tothe inner projecting portion formed at the end portion of thecylindrical holder 4 by a supporting member 6 and which is providedbetween the cathode sleeve 1 and the plurality of straps 5. Heating isperformed by a heater 8 inserted inside the cathode sleeve 1.

The material of the porous cathode substrate 3 is W. Pore portions ofthis base are impregnated with an electron emission substance consistingof a mixture whose mole ratio of BaO:CaO:Al₂ O₃ is 4:1:1.

Note that this cathode assembly is fixed to an insulating supportingmember 10, together with a plurality of electrodes provided sequentiallyat predetermined intervals on the cathode assembly by means of a strap 9attached to the outer surface of the cylindrical holder 4. (Only anelectrode G1 of the first grid is shown in the figure.)

The porous cathode substrate 3 has a structure similar to that shown inFIG. 8, and can be formed by a screen printing method, as will bedescribed below.

At first, W particles, ethyl cellulose as a binder, a mixture of resinand an interface active agent, and a solvent are mixed to prepare acoating solution.

As a large diameter low porosity layer, a tungsten base is preparedwhich, for example, is made of W particles having a particle diameter ofabout 3 μm and has a porosity of about 17%. This base, for example, hasa diameter of 1.1 mm and a thickness of 0.32 mm.

A tungsten thin film layer having a small particle diameter and a highporosity is formed on the base by screen-printing the coating solution,with use of a stainless mesh screen.

Thereafter, sintering is performed for one hour in an atmosphere of H₂at a temperature of 2000° C., for the purpose of sintering the thin filmlayer and of adhering and sintering the thin film layer and the largeparticle diameter low porosity layer.

The obtained tungsten thin film layer having a small particle diameterand a high porosity is not apparently cracked, and has a sufficientstrength, an average particle diameter of 1 μm, a porosity of about 30%,and a uniform thickness of about 10 μm. In addition, the cathodesubstrate thus obtained has the same two-layered structure as that shownin the model view of FIG. 8.

The method as described above was used to form a cathode substrate for acathode ray tube in which the particle diameter and the porosity of thesmall particle high porosity region as well as the particle diameter andthe porosity of the large particle diameter low porosity region arechanged. The emission characteristic of this cathode substrate wasevaluated and the cathode substrate was subjected to a forced life test.A cathode substrate thus prepared used tungsten as its material, and hada diameter of 1.1 mm and a thickness of 0.32 mm. An electron emissionsubstance having a mole ratio of BaO:CaO:Al₂ O₃ =4:1:1 was impregnated.The small particle diameter high porosity region was formed to have athickness of 10 μm, with use of a screen printing method. Further, asputtered film of Ir was formed on this region.

The emission characteristic depending on a duty was evaluated, at ananode voltage 200 V with a heater voltage of 6.3 V, with use of a diodeassembled by installing a heater, an anode, and the like onto thecathode substrate.

A forced life test was carried out under condition that the heatervoltage was 8.5 V and the cathode current was 600 μA, while a cathodeassembly assembled with use of this cathode substrate was mounted on atelevision picture tube having a screen diagonal size of 760 mm. As formeasurement of the emission, a cathode current was measured when aheater voltage was 6.3 V, a voltage of 200 V was applied to the firstgrid, and a pulse of a duty 0.25% was applied.

The results are shown in the following tables 1 and 2.

                  TABLE 1                                                         ______________________________________                                               Large particle diameter                                                                         Small particle diameter                                     low porosity region                                                                             high porosity region                                          Particle            Particle                                                  diameter  Porosity   diameter                                                                             Porosity                                 Sample   (μm)     (%)          (μm)                                                                             (%)                                   ______________________________________                                        1        3        20         1      20                                        2                3                                                                                     20         1                                                                                    25                                 3                3                                                                                     20         1                                                                                    40                                 4                3                                                                                     20         1                                                                                    45                                 5                3                                                                                     20       0.05                                                                                  30                                  6                3                                                                                     20       0.1                                                                                    30                                 7                3                                                                                     20         1                                                                                    30                                 8                3                                                                                     20        1.5                                                                                  30                                  9                3                                                                                     20         3                                                                                    30                                 10              3        10         1                                                                                    30                                 11              3        15         1                                                                                    30                                 12              3        25         1                                                                                    30                                 13              3        30         1                                                                                    30                                 14              1        20         1                                                                                    30                                 15            1.5        20         1                                                                                    30                                 16              2        20         1                                                                                    30                                 17              10                                                                                    20          1                                                                                    30                                 18              15                                                                                    20          1                                                                                    30                                 ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                              Emission  Emission Forced          Total                                      at duty     at duty                                                                                life                  evalu-                       Sample                                                                              0.1% (%)  0.1% (%) (%)    Others      ation                             ______________________________________                                        1     88        88       120             x                                    2           103     128       103                                                                                               ∘               3           103     125       102                                                                                               ∘               4           102     107       100                                                                               Peeling                                                                                  Δ                                                               of small                                                                                          particle                                                                      diameter high                                                                 porosity                                                                      region               5      60       70       120              ΔDifficult                                                                        impregnation              6           100     120       107                                                                                              ∘                7           105     166       101                                                                                              ⊚             8           102     120       101                                                                                              ∘                9           93       75        100                                                                                             x                            10     101      132      69               Δfficult                                                                       impregnation                 11         100      129       93                                                                                                   ∘            12         102      150       90                                                                                                   ∘            13         120      173       40                                                                                                   x                        14         82        121      66                                                                                                   x                        15         82        118      79                                                                                                   Δ                  16         93        105      100                                                                                                  ∘            17         92        102      100                                                                                                  ∘            18         68        88        91                                                                                Difficult                                                                              Δ                                                                                   sintering                                                                     of                    ______________________________________                                                                        substrate                                 

In the tables, values of the emission (%) at a duty of 0.1% are testvalues expressed in percentage with respect to an emission amount as 100(%) which is obtained when pulse operation of a duty 0.1% is performedwith use of an electron tube using a cathode assembly which includes nosmall particle diameter high porosity region and which has a particlediameter of 3 μm and a porosity of 20%. In the same manner, values ofthe emission (%) at a duty of 4.0% are test values expressed inpercentage with respect to an emission amount as 100 (%) which isobtained when pulse operation of a duty 4.0% is performed with use of anelectron tube using a cathode assembly which includes no small particlediameter high porosity region and which has a particle diameter of 3 μmand a porosity of 20%. Further, the forced life (%) is expressed by thefollowing calculation (2).

    (I.sub.life /I.sub.0)/(I.sub.life.sup.ref /I.sub.0.sup.ref)×100 (%)(2)

Here, I₀ ^(ref) is an emission value of an electron tube using a cathodesubstrate which has no small particle diameter high porosity region andwhich has a particle diameter of 3 μm and a porosity of 20% before aforced life test, and I_(life) ^(ref) is an emission value after theforced life test for 3000 hours. Meanwhile, the I₀ is an emission valueof an electron tube using a cathode assembly having a structure shown inthe table before a forced life test, and I_(life) is an emission valueafter a forced life test for 3000 hours.

The forced life test was performed under condition that the cathodefilament voltage was raised to 8.5 V from 6.3 V which is a cathodefilament voltage of a conventional electron tube and the cathodetemperature was kept increased.

As is apparent from the tables 1 and 2, when the porosity is 25 to 40%,the ion-impact resistance is improved. However, it is found that thereis a tendency that the emission characteristic is deteriorated when theporosity is less than 25 and a sufficient strength of the small particlehigh porosity region cannot be obtained. When the particle diameter ofthe small particle diameter high porosity region is 0.1 μm or more andis less than 2 μm, the ion-impact resistance is improved. However, whenthe particle diameter is smaller than 0.1 μm, the number of pores openedin the cathode surface is considerably reduced so that it is difficultto perform impregnation. It is also found that a sufficient ion-impactresistance cannot be obtained when the particle diameter is larger than2 μm.

In addition, when the porosity of the large particle diameter lowporosity region is 15 to 25%, an excellent cathode characteristic isobtained. However, when the porosity is lower than 15%, the amount of animpregnated electron emission substance is apparently reduced so thatthe life-time is shortened. When the porosity exceeds 25%, there is atendency that the evaporation speed of the electron emission substanceis much increased so that the life-time is shortened. When the particlediameter of the large particle diameter low porosity is 2 μm or more andis smaller than 10 μm, an excellent cathode characteristic can beobtained. However, when the particle diameter is smaller than 2 μm,there is a tendency that closed pores appear, the impregnation amount isreduced, the life-time is shortened, and the emission characteristic isdeteriorated. In addition, when the particle diameter of the largeparticle diameter low porosity region exceeds 10 μm, there apparently isa tendency that an enormous energy or time is required to obtain apredetermined porosity by means of sintering.

Embodiment 3

This embodiment shows a third example of an impregnated-type cathodeassembly according to the present invention.

At first, a porous W base was prepared as a large particle diameter lowporosity layer similar to that of the embodiment 1. A plurality ofprocessing grooves were formed to be 20 to 50 μm deep and at an equalpitch of 20 to 50 μm, in the surface of the porous W base, by means ofmechanical processing such as grinding. Thereafter, W powder having anaverage particle diameter of 0.5 to 1 μm was filled in the processinggrooves.

Thereafter, a heat treatment was performed in the same manner as in theembodiment 1. A model view of a cathode substrate thus obtained is shownin FIG. 11. As shown in FIG. 11, this cathode substrate comprises amatrix consisting of a porous W base 42 as a large particle diameter lowporosity region which is made of W particles of an average particlediameter of about 3 μm and which a porosity of about 17%, and W regions41 which are scattered in the surface of the substrate and which have anaverage particle diameter of 0.5 to 1 μm and a porosity of 30%.

Embodiment 4

This embodiment shows a fourth example of an impregnated-type cathodeassembly according to the present invention. Here, a cathode substrateused for a cathode assembly of the same type as the embodiment 2 wasformed by a spraying method.

At first, a porous W base which has a shape similar to that of theembodiment 2, a particle diameter of 3 μm, and a porosity of 20% wasprepared as a large particle diameter low porosity layer.

In the next, a mixture of W particles and butyl acetate was prepared asa coating solution. This coating solution was vertically sprayed to thesurface of the base, with use of an air-gun, at a spraying distance of10 cm with an air pressure of 1.2 kg/cm² at a spray flow amount of 0.35cc/sec for a spraying time of 5 seconds. A coated film thus obtained wasdried thereafter, and was subjected to a heat treatment for ten minutesin a hydrogen atmosphere at a temperature of 1900° C. for the purpose ofsintering the coated film and adhering the same to the substrate. A thinfilm of W thus formed and having a small particle diameter and a highporosity was not apparently cracked, and had a sufficient strength, afilm thickness of 20 μm, an average particle diameter of 1 μm, and aporosity of 30%. In addition, the structure of the cathode assembly wasthe same as that shown as a model view of FIG. 8.

As shown in FIG. 8, an electron emission substance consisting of amixture whose mole ratio of BaO:CaO:Al₂ O₃ was applied onto the cathodesubstrate 23 having the two-layered structure, and was heated for tenminutes in a H₂ atmosphere at a temperature of 1700° C., so that theelectron emission substance was melted and impregnated as shown in FIG.24.

The cathode assembly thus prepared was adopted in the impregnated-typecathode assembly as shown in FIG. 10, and was equipped with an anode,thus preparing an electron tube of a diode structure. The electronemission characteristic of this electron tube was measured. As a resultof this, the tube according to the present invention is improved in theelectron emission characteristic in a high duty range in comparison witha conventional impregnated-type cathode.

Embodiment 5

This embodiment shows a fifth example according to an impregnated-typecathode assembly of the present invention.

Here, the method of forming a thin film layer of W having a smallparticle diameter and a high porosity is as follows.

Except that W particles and a mixture solution of diethyl carbonate andnitrocellulose were prepared as a coating solution and that this coatingsolution was applied to the same porous W substrate as that of theembodiment 4 rotated at a speed of 1000 rpm by a spin-coating method,thin film layers of various thicknesses each having a small particlediameter and a high porosity were formed in the same manner as in theembodiment 4, and a cathode substrate was thus obtained. The thin filmlayer had an average particle diameter 1 μm and a porosity of 30%. Thecathode substrate thus obtained had a two-layered structure as shown inFIG. 8.

An electron emission substance was melted and impregnated into thecathode substrate, in the same manner as in the embodiment 4.

In the next, a thin film layer of Ir was formed in the electron emissionsurface side of the cathode substrate impregnated with the electronemission substance, by a sputtering method. To form an alloy from an Irthin film layer thus obtained and W of the cathode substrate, thecathode substrate on which an Ir film was formed was subjected to a heattreatment for 10 minutes in a hydrogen atmosphere at a temperature of1290° C.

The electron emission characteristic of an impregnated-type cathode thusobtained was evaluated in the same manner as in the embodiment 4. FIG.12 shows the relationship between the duty of an applied pulse and theemission change rate, in this evaluation.

FIG. 12 shows the relationship between the duty and the emission changerate with respect to a case in which no small diameter high porositylayer was included in the two-layered structure and a case in which thelayer thickness of the small diameter high porosity layer was changed.In this figure, a solid line 100 indicates a case of including no smallparticle diameter high porosity layer, a solid line 103 indicates a caseof adopting a film thickness of 3 μm, a solid line 110 indicates a caseof adopting a film thickness of 10 μm, a solid line 120 indicates a caseof adopting a film thickness of 20 μm, and a solid line 130 indicates acase of adopting a film thickness of 30 μm. In this embodiment, thelarge particle diameter low porosity layer had a particle diameter of 3μm and a porosity of 20%, and the small particle diameter high porositylayer had a particle diameter of 1 μm and a porosity of 30%. Inaddition, the emission change rate is expressed, with an emissionobtained at a duty of 0.1% being regarded as 100%. The measurementconditions were a heater voltage of 6.3 V and an anode voltage of 200 V.

As is apparent from this figure, according to the present invention, theelectron emission characteristic is improved in a high duty range, incomparison with a conventional cathode assembly, and an excellentelectron emission characteristic in a high duty range can be obtainedwhen the film thickness is within a range of 3 to 30 μm.

Embodiment 6

This embodiment shows a sixth example of an impregnated-type cathodeassembly according to the present invention.

At first, a porous W substrate having a particle diameter of 3 μm and aporosity of 20% was prepared as a large particle diameter low porositylayer. This cathode substrate is applicable to the cathode assembly fora cathode ray tube as shown in FIG. 10. W particles together with anorganic solvent were prepared like paste on the electron emissionsurface layer of the cathode substrate, and was coated by screenprinting such that a mixture layer had a thickness of 20 μm. Thereafter,coated paste was dried and subjected to a heat treatment for ten minutesin a hydrogen atmosphere at 1900° C., thereby to form a thin film layerof W having a small particle diameter and a high porosity. Note that thedensity of paste W, printing conditions, and the sintering time andtemperature were arranged such that a sintered porous layer has anaverage particle diameter of 1 μm and a porosity of 30%.

A cathode substrate thus prepared had a two-layered structure as shownin FIG. 8.

An electron emission substance made of a mixture whose mole ratioBaO:CaO:Al₂ O₃ was 4:1:1 was adopted, and this substance was melted andimpregnated in pore portions of the cathode substrate, in a hydrogenatmosphere at a temperature of 1700° C. for 10 minutes.

Two layers of ScH₂ layers as Sc compound thin film layers and Re layersas high melting point metal thin film layers were alternately formed onthe surface of the cathode substrate thus formed, by a sputteringmethod.

The cathode substrate thus obtained had a structure in which a smallparticle high porosity layer 23 was layered on a large particle diameterlow porosity layer 22, as shown in FIG. 13, and ScH₂ layers 25 and 27and Re layers 26 and 28 as high melting point metal thin film layers arealternately layered on the layered assembly whose pores are impregnatedwith an electron emission substance. Each of the ScH₂ thin film layersand Re thin film layers had a thickness of 20 nm, and sputtering wasalternately performed on every two of these layers. In particular, whensputtering ScH₂ thin film layers, a H₂ gas was introduced in addition toan Ar gas in order to prevent separation of H₂.

The cathode assembly thus prepared was adopted in an impregnated-typecathode assembly as shown in FIG. 10 and was equipped with an anode. Anelectron tube having a diode structure was thus prepared. The electronemission characteristic of this electron tube was evaluated as follows.At first, a pulse of 200 V was applied between the cathode and anode, ata heater voltage of 6.3 V. Here, while the duty of an applied pulse waschanged from 0.1 to 9.0%, the emission current density was measured.

FIG. 14 is a graph showing the emission. characteristic of theimpregnated-type cathode according to this embodiment, in form of arelationship between the duty and the emission current density of theimpregnated-type cathode. In this figure, the curve 71 indicates ameasurement result of a conventional (top-layer scandate) cathodesubstrated on, the curve 72 indicates a measurement result of aimpregnated-type cathode according to the present invention, and thecurve 73 indicates a measurement result of a conventional metal-coatedimpregnated-type cathode. The impregnated-type cathode according to thepresent invention has a more excellent emission current characteristicin both of low and high duty ranges than that of a conventionalimpregnated-type cathode.

When Ru or Hf was used as another example in place of Re contained inthe high melting point metal thin film layer, or when Sc was used inplace of ScH₂ contained in the scandium compound thin film layer, thesame characteristic as described above was obtained.

Embodiment 7

This embodiment shows a seventh example of the present invention.

FIGS. 15 to 21 are views for explaining steps of manufacturing a cathodesubstrate used in the present invention.

At first, tungsten particles having an average particle diameter of 3 μmwere used to obtain a porous substance of a large particle diameter lowporosity layer having a porosity of 20% in a normal method.

Thereafter, a film of paste containing tungsten was formed on a screenprinting method, on the large particle diameter low porosity layer asobtained above. Subsequently, the film of paste was sintered for 30minutes in a hydrogen atmosphere at a temperature of 1800° C., therebyobtaining a small particle diameter high porosity layer of a poroussubstance having an average particle diameter of 1 μm and a porosity of30%. A cathode substrate was thus obtained.

FIG. 15 is a model view showing the cross-sectional structure of thiscathode substrate. As shown in FIG. 15, an obtained cathode substrate123 comprises a large particle diameter low porosity layer 121 and asmall particle diameter high porosity layer formed on the layer 121.

In the next, copper particles were used to form a copper particle layer131 on the large particle diameter low porosity layer 121. As a methodof forming the copper particle layer 131, it is possible to use a methodof performing screen printing with use of paste containing copperparticles, and a method of directly covering the small particle highporosity layer 122 with copper particles. Here, the method of directcovering was used.

FIG. 16 is a model view showing a cross-sectional structure of thecathode substrate thus obtained. As shown in FIG. 16, the cathodesubstrate 133 using copper particles had a copper particle layer 131 onthe cathode substrate 123.

Thereafter, the cathode substrate 133 was set in a cup made ofmolybdenum, and heated to a temperature of 1080° C. in a hydrogenatmosphere, thereby melting the copper particles 131 and covering thesurface of the small particle high porosity layer 122 with a coppercovering layer. In this state, the heating temperature may be 1083° C.at most, and can be set to a temperature within a range in which coppercovering can be sufficiently carried out.

FIG. 17 is a model view showing a cross-sectional structure of thecathode substrate 143 covered with a copper cover layer. As shown inFIG. 17, the cathode substrate 143 is covered with a copper cover layer141.

FIG. 18 is a schematic view for explaining a step of cutting the cathodesubstrate. As shown in FIG. 18, an obtained cathode substrate 143 wasthereafter cut by a laser beam 151 from a laser light source 150, andwas cut into respective pieces of cathode substrates each having apredetermined size, as shown in FIG. 19.

FIG. 20 is a view showing the shape of a piece of the cathode substratecut out as described above. FIG. 21 is a view schematically showing thestate of the cathode substrate after tumbling processing. As shown inFIG. 20, a cut-out cathode substrate 160 had burrs 161, andcontaminations 162 or the likes stick to the substrate 160 due tooxidization and evaporation.

Further, the cathode substrate 160 thus cut out was put in a closedcontainer, together with a ball made of alumina and silica, and tumblingprocessing was performed with use of a barrel polisher. As shown in FIG.21, burrs 161 and contaminations 162 were removed through thisprocessing, so that a cathode substrate 180 comprising a large particlediameter low porosity layer 121, a small particle diameter high porositylayer 122, and a copper cover layer 141 was obtained.

The cathode substrate 180 thus obtained was dipped in a solution whosevolume ratio of nitric acid:water is 1:1 for 12 hours, and wasthereafter dried. Thereafter, the cathode substrate 180 was set in a cumade of molybdenum, and was heated at 1500° C. until flame of copperceased. Copper was thus removed. FIG. 22 is a model view showing a stateof a cathode substrate from which copper was removed. As shown in FIG.22, deterioration in the shape of the surface due to cutting andtumbling was not found on the surface of the small particle diameterhigh porosity layer 122 after removal of copper, and thus, the surfacecondition was excellent. In addition, blockage of pore portions of thesmall particle diameter high porosity layer 122 was not found.

Subsequently, an electron emission substance obtained by mixing bariumoxide, calcium oxide, and aluminum oxide at a mole rate of 4:1:1 wasapplied onto the surface of the small particle high porosity layer 122,and was heated at a temperature of 1650° C. for three minutes in ahydrogen atmosphere, so that the substance was melted and impregnatedinto the cathode substrate 180. FIG. 23 is a model view showing thestructure of an impregnated-type cathode thus obtained. As shown in FIG.23, the applied electron emission substance 208 was impregnated into thepore portions of the large particle diameter low porosity layer 121through the pore portions of the small particle diameter high porositylayer 122.

As explained above, according to the seventh example, cutting andtumbling steps are improved by using the method of the presentinvention, so that an excellent impregnated-type cathode can beobtained.

Embodiment 8

The following explains an eighth example of the present invention.

FIGS. 24 and 25 are views explaining manufacturing steps of a cathodeassembly used in the present invention.

At first, a large particle diameter low porosity layer having an averageparticle diameter of 3 μm and a particle of 20% was obtained in the samemanner as in the embodiment 7.

Thereafter, paste containing tungsten powder and copper particles wasused to form a film on the large particle diameter low porosity layer asobtained above, by a screen printing method. Subsequently, the film ofpaste thus formed was sintered for 30 minutes at 1800° C. in a hydrogenatmosphere, and thus, a cathode substrate made of a porous body of asmall particle diameter high porosity layer having an average particlediameter of 1 μm and a porosity of 30% was obtained.

FIG. 24 is a model view showing a cross-sectional structure of thecathode substrate. As shown in FIG. 24, a cathode substrate 213 thusobtained had a two-layered structure consisting of a large particlediameter low porosity layer 211 and a small particle diameter highporosity layer 212, wherein the small particle diameter high porositylayer 212 was a porous layer containing tungsten particles 214 andcopper particles 215.

By heating the cathode substrate 213 in the same manner as in theembodiment 7, copper particles 131 were melted and the surface of thesmall particle diameter high porosity layer 212 was covered with copper,thus filling the pore portions with copper.

FIG. 25 is a model view showing a cross-sectional structure of a cathodesubstrate in which pore portions were filled with copper. As shown inFIG. 25, the small particle diameter high porosity layer 222 of thecathode substrate 223 had a structure in which pore portions betweentungsten particles 214 were filled with melted copper 225.

The cathode substrate 223 thus obtained was cut in the same manner as inthe embodiment 7, and tumbling processing was carried out to removecopper components. Deterioration in the shape of the surface due tocutting and tumbling was not found in the surface of the small particlediameter high porosity layer after copper was removed, and the surfacecondition was excellent. In addition, blockage of the pore portions ofthe small particle diameter high porosity layer was not found.

Subsequently, an electron emission substance was applied and melted ontothe surface of the small particle diameter high porosity layer, in thesame manner as in the embodiment 7, and thus, the substance can besufficiently melted and impregnated into the cathode substrate.

According to the eighth embodiment, cutting and tumbling steps areimproved by using the method of the present invention, so that anexcellent impregnated- type cathode can be obtained without makingdamages on the electron emission surface.

The impregnated-type cathode substrate or the impregnated-type cathodeassembly using the substrate was used for electron tubes, such as acathode ray tube, a klystron tube, a traveling-wave tube, and agyrotron, e.g., the cathode ray tube shown in FIG. 3, the klystron tubeshown in FIG. 4, the traveling-wave tube shown in FIG. 5, and thegyrotron shown in FIG. 6. Then, various electron tubes were obtainedwhich attains a high performance ability and a long life time and whichhave a sufficient ion-impact resistance and an excellent electronemission characteristic, under condition of a high voltage and a highfrequency. Note that the impregnated-type cathode substrate of thepresent invention is not limited to the embodiments as described above,but may be used for other various electron tubes.

We claim:
 1. An impregnated-type cathode substrate comprising a largeparticle diameter low porosity region and a small particle diameter highporosity region which is provided in a side of an electron emissionsurface of the large particle diameter low porosity region and has anaverage particle diameter smaller than an average particle diameter ofthe large particle diameter low porosity region and a porosity higherthan a porosity of the large particle diameter low porosity region, saidimpregnated-type cathode being impregnated with an electron emissionsubstance.
 2. An impregnated-type cathode substrate according to claim1, wherein the large particle diameter low porosity region has anaverage particle diameter of 2 to 10 μm and a porosity of 15 to 25%. 3.An impregnated-type cathode substrate according to claim 1, wherein thesmall particle diameter high porosity region has an average particlediameter of 0.1 to 2.0 μm and a porosity of 25 to 40%.
 4. Animpregnated-type cathode substrate according to claim 1, wherein thesmall particle diameter high porosity region has a thickness of 30 μm orless.
 5. An impregnated-type cathode substrate according to claim 1,wherein the small particle diameter high porosity region is providedlinearly or scattered in the electron emission surface side of the largeparticle diameter low porosity region.
 6. An impregnated-type cathodesubstrate according to claim 1, wherein the impregnated-type cathodesubstrate has an average particle diameter and a porosity which changein steps from the large particle diameter low porosity region to thesmall particle diameter high porosity region.
 7. An impregnated-typecathode substrate according to one of claims 1 to 3, wherein a layerincluding at least one kind of metal selected from a group of iridium,osmium, rhenium, ruthenium, rhodium, and scandium is formed on theelectron emission surface.
 8. An impregnated-type cathode assemblyincluding an impregnated-type cathode, comprising: a large particlediameter low porosity region and a small particle diameter high porosityregion which is provided in a side of an electron emission surface ofthe large particle diameter low porosity region and has an averageparticle diameter smaller than an average particle diameter of the largeparticle diameter low porosity region and a porosity higher than aporosity of the large particle diameter low porosity region, saidimpregnated-type cathode being impregnated with an electron emissionsubstance.